1. Field of the Invention
The present invention relates to a silicon nitride sintered body comprising silicon nitride as the principal component and silicon carbide dispersed therein, which is suitable for use in a structural material and the like.
2. Description of the Related Arts
A silicon nitride sintered body comprising silicon nitride (hereinafter referred to simply as "Si.sub.3 N.sub.4 ") as the principal component is used as a structural material and the like, for example, gas turbine materials, heat exchanger materials, bearings, etc., because of its high strength and excellent thermal shock resistance and corrosion resistance.
Particularly among the examples enumerated above, materials for use at high temperatures of 1,300.degree. C. or higher, such as gas turbine materials, require further improved high-temperature characteristics such as high strength and creep resistance at high temperatures.
Creep resistance is evaluated by the degree of progress in fracture, more specifically, by the duration until fracture and the amount of deformation of the object when an object such as a sintered body is kept with a load (stress) applied thereto. An object is evaluated as having a high creep resistance when the duration until fracture is long and the deformation until fracture (for instance, the elongation at fracture when a tension is applied) is small.
In particular, in the case that a sintered body is used as a high-temperature material in a component for an engine such as a gas turbine, a constant load (stress) is applied to the sintered body for a long duration of time at a high temperature. Accordingly, the sintered body is required to have a resistance against the applied load; that is, an improved creep resistance of the sintered body at high temperatures is required.
As a means for improving the high-temperature characteristics of a silicon nitride sintered body, it has been proposed conventionally to disperse silicon carbide (hereinafter referred to simply as "SiC") grains in the Si.sub.3 N.sub.4 sintered body. A silicon nitride sintered body with improved high-temperature strength, creep resistance, and oxidation resistance can be obtained by dispersing SiC grains therein.
More specifically, a silicon nitride sintered body with superior toughness and hardness can be implemented by adding and dispersing SiC grains in the silicon nitride sintered body. In this manner, a silicon nitride sintered body with improved wear resistance can be realized. When fine SiC grains are added into a silicon nitride sintered body, they reside inside the grains of Si.sub.3 O.sub.4 or in the grain boundaries. These fine SiC grains prevent grain boundary sliding from occurring, and improve the high-temperature characteristics of the silicon nitride sintered body. The fine SiC grains may be spherical grains, platelets, whiskers, long fibers, etc.
Some examples of the silicon nitride sintered body in which the fine SiC grains are dispersed are given below.
A silicon nitride sintered body with fine SiC grains dispersed therein is obtained by mixing Si.sub.3 N.sub.4, SiC, and a sintering aid as the starting materials and then sintering the mixture by means of hot pressing or by encapsulated HIP (Hot Isostatic Pressing using a capsule). Another example, as disclosed in JP-A-2-160669 (the term "JP-A" as referred to herein signifies an "unexamined Japanese patent application"), provides a silicon nitride sintered body in which fine SiC grains of 1 mm or less in average diameter are dispersed in a Si.sub.3 N.sub.4 matrix. The sintered body is obtained by adding a sintering aid such as Y.sub.2 O.sub.3 to synthetic amorphous Si-C-N powder prepared by CVD (chemical vapor deposition) process and then sintering the mixture by hot pressing or the like.
However, the prior art silicon nitride sintered bodies containing SiC have insufficient high-temperature characteristics such as high-temperature strength. Otherwise, even though they may have good high-temperature characteristics such as a strength of, for example, 700 MPa or higher at a temperature of 1,300.degree. C., they are manufactured only by hot pressing which results in poor productivity. For instance, the sintered body disclosed in JP-A-2-160669 referred to hereinbefore yields a maximum strength of 121 kg/mm.sup.2 (as obtained by three-point bending test) at 1,200.degree. C. However, no data for the strength at a temperature 1,300.degree. C. or higher is available. Even though a high strength sintered body is obtained, JP-A-2-160669 discloses that the sintering process should be carried out by hot pressing. It can be seen, therefore, that the process is not feasible from a practical point of view.
Furthermore, JP-A-60-46973 discloses a sintered body of Si.sub.3 N.sub.4 and SiC in which from 0.02 to 50.00 atomic percent of an oxide, such as boron oxide, is contained. The content of a boron compound in the sintered body as disclosed in Example 5 of JP-A-60-46973 is found to be 0.53% by weight in terms of the weight of boron (B). However, the addition of boron into the silicon nitride sintered body in such a large quantity results in the formation of a low-melting B.sub.2 O.sub.3 -based glassy phase which greatly impairs the high-temperature characteristics, particularly, the creep resistance, of the sintered body.